[something]) in Asterisk are the correct name for configuration sections. A context is a dialplan-specific terminology. An IAX2 user is defined as a section in iax.conf, and the configuration for this user specifies a context (or multiple contexts) in the dialplan in which to begin dialplan execution when a call is received on this user (a similar thing can be done for peers on outgoing calls in the configuration, or in the dial string).
from-phreaknetis the default. This "default" or implicit dialplan context is not specified as part of routing, which means you can actually change the name of this if you want. Other contexts, such as
from-stepnet, are used as part of routing, so these names can't be changed.
PhreakNet will complete any calls begining with 101-XXXX to the carrier designated by XXXX, including any additional digits following. This means that dialing 101-XXXX can be used to access a specific Feature Group D carrier's dial tone, dialing 101-XXXX-0 can be used to reach a specific Feature Group D carrier's operator, and dialing 101-XXXX-NNX-XXXX could be used to access a specific number through a Feature Group D carrier directly. Dialing fewer digits will "cut through", allowing the carrier to receive additional digits directly. It is up to each Feature Group D facility to properly handle any scenarios that should be supported and to inform users of dialing procedures.
exten => 9501234or
exten => _1010222!(for Feature Group D, you should capture any digits that begin with your carrier access code, as additional digits may or may not be delivered to you initially).